Three more former aides of Zhou Yongkang have been expelled from the Communist Party for graft and are now facing prosecution.

The move against officials who were close to Zhou indicate the investigation into the retired security chief himself may soon reach a conclusion.

"I think there will be a conclusion before the fourth plenary meeting of the Party's Central Committee," said Zhang Lifan, a Beijing political commentator.

Ji Wenlin, a close aide of Zhou's and a former deputy governor of Hainan, and Yu Gang, ex-deputy director of the general office of the Politics and Legal Affairs Commission, are two of those expelled. They both served as secretaries to Zhou.

Yu was Zhou's last secretary at the Politics and Legal Affairs Commission, which was led by Zhou from 2007 to 2012.

Ji was removed from his post in Hainan on March 27. The anti-corruption watchdog had announced in February that he was being investigated for "serious disciplinary violations".

The two were also accused of having extramarital affairs, according to the Central Commission for Discipline Inspection. Communist Party members can be punished for immoral behaviour, such as adultery.

The third disgraced official, Tan Hong, was a senior member of the security protection bureau in the Ministry of Public Security. Zhou was the head of the ministry from 2002 to 2007.

Zhou is under disciplinary investigation. His power bases spanned domestic security agencies, the state petroleum sector and Sichuan province, where he was provincial party boss.

On Monday, the party announced the expulsion of another three Zhou aides: Jiang Jiemin, the former chief regulator of state-owned enterprises and a former executive of China National Petroleum Corporation (CNPC); Li Dongsheng, the former vice-minister of public security; and Wang Yongchun, a former deputy general manager of CNPC.

The Politburo also said on Monday that former PLA general Xu Caihou - the retired vice-chairman of the Central Military Commission - had been expelled and was facing prosecution for corruption. Xu was an ally of Zhou.

"The shock and fear from all these investigations may keep many awake at night," commentator Zhang said.